This invention relates generally to measuring liquid levels in containers that may either underground or on the surface.
U.S. Pat. No. 2,069,793 (Watson) discloses a liquid level sensing device that employs a sequence of angled slots distributed along its length to control the movement of a float locking surface that secures the float, which rests on the liquid-air interface in position against a scale. This disadvantage of this approach is that frictional forces between the slot surfaces and the moving element in the slot limit the uniformity of movement of the locking surface, which results in ambiguous liquid level readings. In addition, the cumulative effect of the frictional forces requires that considerable force be applied to the float locking/unlocking mechanism.
U.S. Pat. No. 7,043,984, which issued to the present inventors on May 16, 2006, attempts to compensate for the variable friction forces in the slot mechanisms by providing a sequence of springs along the length of a frame. The springs apply forces to a float locking member that assures closer tolerance control of the spacing between the float locking surface and the interior wall of the frame of the liquid level sensing device.